Process for the production of polarizing glass

ABSTRACT

A process for producing a polarizing glass containing shape-anisotropic metallic particles dispersed in an oriented state therein, which comprises drawing a glass preform containing metallic halide particles dispersed therein while its viscosity being held above 2×10 6 , but below 7×10 7  poises; and subjecting the drawn glass to a reducing treatment so that a part or all of the metallic halide particles are reduced to metallic particles, which process enables it to produce a polarizing glass with a high yield from a starting material of a glass containing metallic halide particles, while avoiding glass to breakage or fracture during elongation as well as preventing the elongated metallic halide particles from returning to a spherical shape.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing polarizingglass articles. More specifically, the present invention relates to aprocess for producing infrared polarizing glass articles containing finemetallic particles having an anisotropic shape and being excellent inpolarizing properties (with an extinction ratio of 40 dB or more). Thearticles can be used as polarizers utilized in miniature opticalisolators adapted for optical communication, optical switches comprisinga liquid crystal, an electro-optical crystal, a Faraday rotator etc., aswell as electromagnetic sensors.

Glass articles containing shape-anisotropic fine metallic particles,such as silver or copper, dispersed in an oriented state therein havebeen known to behave as optical polarizers. In such glasses, thewavelength of the resonance absorption peak of the metal particles canvary depending upon the direction of incident polarized light. It hasalso been known that such polarizing glasses could be produced byreducing glasses containing elongated copper halide or silver halideparticles.

Silver-free polarizing photochromic glasses and a production processthereof have been disclosed in U.S. Pat. No. 3,954,485. The disclosedglasses show polarizing properties in the darkened state ofphotochromic. The polarizing properties, however, are obtainable only inthe darkened state and the resulting extinction ratio is as low as 10dB, which is too low to apply them as optical isolators. According tothe process described in the patent, starting glasses are stretched in atemperature region that imparts a viscosity of 1×10⁷ to 1×10⁹ poises to100 to 1000 Å particles in the secondary phase (separated borate-richphase) containing 20 to 50 Å copper halide/cadmium phase, so that thesecondary phase can be elongated to have an aspect ratio of 2:1 to 5:1whereby polarizing photochromic glasses are produced. However, when theintended polarizing glasses are to be produced by starting with a glasscontaining metallic halide particles, elongation of the particles isdifficult and the elongated particles easily return to a sphericalshape. This is because the interfacial energy generated between metallichalide particles and glass is higher than that generated between thesecondary phase and glass on one hand and an aspect ratio of 10:1 ormore is required on the other hand. Thus, it is impossible to elongatemetallic halide particles at a viscosity of less than 1×10⁸ poises.

A process for production of polarizing glass by starting with a glasscontaining copper halide particles is disclosed in the Japanese PatentApplication Laid Open (JP-A-) No. 5-208844. The disclosed processcomprises the steps of elongating copper halide particles in the glassby pulling or extruding the glass containing copper halide particles ata temperature at which the glass exhibits a viscosity of 10⁸ to 10¹¹poises and then heat treating the elongated glass under a reducingatmosphere to reduce the copper halide particles whereby a polarizingglass containing elongated shape-anisotropic metallic copper particlesis produced.

A process for production of polarizing glass by starting with a glasscontaining silver halide particles is disclosed in the Japanese PatentApplication Laid Open (JP-A-) No. 59-83951. Also in the process, theelongation of silver halide particles is effected by pulling orextruding the glass at a temperature at which the glass exhibits aviscosity of 10⁸ to 10¹³ poises, in a manner substantially similar tothat of the glass containing copper halide particles.

In these glass stretching processes cited above, proper workingtemperature and cooling procedure should be applied so as to prevent theelongated particles from returning to a spherical shape. This isbecause, when the glass is elongated at a temperature at which the glassexhibits a viscosity lower than 10⁸ poises, the elongated particles arelikely to return to a spherical shape and thus it is substantiallyimpossible to obtain a polarizing glass.

Moreover, in the processes mentioned above, a high stress is required soas to achieve the elongation of silver halide particles. Such highstress, however, may exceed the practical maximum breaking stress of theglass when applied to the glass in the above-defined viscosity range.Hence, the glass would frequently break or fracture during theelongation step irrespective of the nature of the particles containedtherein. The same problem occurs in the process of the Japanese PatentApplication Laid Open (JP-A-) No. 5-208844 comprising the elongation ofcopper halide particles to obtain a polarizing glass. The break orfracture of the glass during the elongation step may remarkably decreasethe factory production efficiency of the polarizing glass, thus shouldnot be neglected from the viewpoint of practical use.

Under these circumstances, the above-cited Japanese Patent ApplicationLaid Open (JP-A-) No. 59-83951 has disclosed a process for drawing acomposite (laminated) glass, according to which the glass particles maybe elongated without breaking the glass.

The glass stretching step of the drawing process described in thisdocument is schematically shown in FIG. 2. In FIG. 2, a glass to beredrawn is composed of a potential polarizing core glass 9 and a surfaceglass 10. A blank 8 is passed through a heater part of a redrawingfurnace indicated by the arrow 11, where it is heated and elongatedunder tension applied by pulling rolls 12, whereby the blank can beconverted to a laminated polarizing glass sheet or strip 13. Thisdocument reports that if a silver halide-containing glass is subjectedto redrawing or stretching under the conditions under which the glassexhibits a viscosity of about 10⁸ poises, a polarizing glass could beproduced without giving rise to glass break or fracture during thestretching step. According to this process, in order to avoid breakingof the stretched article, the potential polarizing core glass 9 iscoated with glass 10 which exhibits a considerably low viscosity so thatthere remains scarcely any tensile stress on the surface of the glass.

However, when a laminated polarizing glass is to be produced accordingto the process mentioned above, the first problem is that there wouldstill remain the glass having a low viscosity in the form of a surfacelayer even after the completion of the glass elongation. Hence, it wouldtake a lengthy period of time to reduce the metallic halide particlescontained in the core glass. In fact, the reduction of the metallichalide particles in the core glass can be started only after a gaseousreducing agent has passed through the surface layer.

The second problem is that, even if the glass could be prevented frombreaking, since a glass containing metallic halide particles is coatedwith a superficial glass having a low viscosity, the elongated glasswould be difficult to be cooled efficiently and hence the elongatedparticles will still attain the tendency of returning to a sphericalshape.

That is, there has been as yet unknown a process for efficientlyproducing a polarizing glass while avoiding glass to breakage orfracture during the elongation step and preventing the elongatedparticles from returning to a spherical shape as well.

In laboratory scale, a polarizing glass containing shape-anisotropicmetallic particles is produced by pulling a glass containing metallichalide particles in the form of rod or plate from both ends. With such aprocess, however, it is only in the central part of the elongated samplethat particles elongated to the desired state (aspect ratio) can beobtained and thus this process is unable to be practically used as suchfrom the viewpoint of production efficiency.

To be practical, a starting glass containing metallic halide particlesshould be converted into a glass containing elongated metallic halideparticles while minimizing glass loss.

Moreover, in order to avoid the insertion loss and the deformation oftransmitted light beam, the light receiving face and the light emittingface of a polarizing glass should be polished to improve the surfaceprecision. Conventionally, a glass preform containing metallic halideparticles dispersed therein had been stretched without surfacepolishing, then both of the light receiving face and the light emittingface have been polished prior to reduction treatment.

In such a process, however, it might happen that a glass body thinnerthan a few mm thick is likely to deform when polished and hence it isexpensive to polish such a glass precisely,

Therefore, an object of the present invention is to provide a processfor production of a polarizing glass article which is capable ofproducing a polarizing glass from a starting material of a glasscontaining metallic halide particles with a high yield, while avoidingglass breakage or fracture during the elongation step as well aspreventing the elongated metallic halide particles from returning to aspherical shape.

Another object of the present invention is to provide a process for theproduction of a polarizing glass having a high surface precision from astarting material of a glass containing metallic halide particles with ahigh yield, while avoiding glass breakage or fracture during theelongation step as well as preventing the elongated metallic halideparticles from returning to a spherical shape.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a process forproducing a polarizing glass article containing metallic particlesexhibiting an anisotropic shape dispersed in an oriented state therein,which process comprises the steps of:

drawing a glass preform containing metallic halide particles dispersedtherein while its viscosity is held above 2×10⁶, but below 7×10⁷ poises;and

subjecting the drawn glass to reducing treatment so that a part or allof the metallic halide particles are reduced to metallic particles.

There is also provided the process for producing the polarizing glass asdefined above, wherein the drawing step is effected on a glass preformhaving at least two polished faces opposite to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a schematic view of a drawing apparatus applicable tothe process of the present invention.

FIG. 2 represents a schematic view showing the glass stretching step ofprior art.

FIG. 3 is a graph representing relationship of the thickness versusextinction ratio of polarizing glass.

FIG. 4 is a graph representing the Gaussian temperature distributionobserved in a drawing furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more detail hereinafter.

The process of the present invention is intended to produce a polarizingglass article which contains shape-anisotropic metallic particlesdispersed in an oriented state therein. The term "shape-anisotropy"herein means that the particles have an aspect ratio of more than 1.Relevant metals are copper, silver, gold, platinum and the like. Aspectratio of metallic particles may suitably be selected depending on thephysical properties required for a polarizing glass and may be in therange of from 2:1 to 100:1, for example. In the polarizing glass of thepresent invention, shape-anisotropic metallic particles are dispersedwhile substantially uniaxially oriented. Size and amount of metallicparticles to be dispersed in the glass as well as the nature of theglass serving as matrix are not particularly limited. These may beselected as appropriate depending on the physical properties requiredfor polarizing glass products. By way of example, particle diameter(short diameter) of metallic particles may be in the range of, e.g., 6nm to 150 nm. Amount of the dispersed metallic particles may be in therange of 2×10⁻⁴ mm to 2×10⁻⁶ mm expressed as the volume ratio multipliedby the thickness of reduction layers of a polarizing glass. Examples ofthe glasses used as matrix include silicate glasses, borosilicateglasses, borate glasses and the like.

According to the present invention, the starting material consists of aglass which contains metallic halide particles dispersed therein.Halogens suitable for forming the metallic halides include chlorine,bromine and iodine. Metallic halides which may be mentioned are silverchloride, silver bromide, silver iodide, copper chloride, copperbromide, copper iodide, gold chloride, gold bromide, gold iodide,platinum chloride, platinum bromide, platinum iodide and the like.Glasses which contain metallic halide particles dispersed therein may beeasily produced by using any of conventional processes.

The above-mentioned glass should be subjected to the drawing whilemaintaining at a temperature corresponding to its viscosity in the rangeof 2×10⁶ to 7×10⁷ poises. The drawing of a glass having a viscosity inthe above-defined range enables the glass to be elongated while avoidingglass breakage. As is generally known, stress is required for elongatingmetallic halide particles. The stress to be applied to the particlesunder the conditions under which the glass is deformable is determineddepending on the glass viscosity and the drawing speed.

A viscosity of less than 2×10⁶ poises is too low to apply the stressrequired for the elongation of metallic halide particles. At such a lowviscosity, the metallic halide particles may reach the temperatureregion suitable for the heat treatment to make them precipitate, whichmay disadvantageously cause the change in dimension (particle diameter)of the metallic halide particles during the drawing step. Thus, suchlower viscosity is not preferred.

On the other hand, when the viscosity of a glass is more than 7×10⁷poises, a higher tension may be required for the drawing even when alower drawing rate is used, which may elevate the probability of glassbreaking during the drawing step. Thus, such highly viscosity is notpreferred as well.

The glass subjected to drawing has preferably a viscosity of 4×10⁶ to4×10⁷ poises. Since the viscosity of a glass could vary with the natureand the temperature thereof, a temperature adequate to impart theabove-defined viscosity to the glass should be selected depending on thenature of a glass.

In the process of the present invention, a glass is stretched bydrawing. By means of the drawing, a lengthy glass uniform in size may beobtained and a polarizing glass having higher polarizing properties canbe produced.

It is known that a drawing furnace should have usually a Gaussiantemperature distribution in the drawing direction. The Gaussiantemperature distribution means a temperature distribution resembling toa curve traced by the formula: T=T₀ exp(-aH²), wherein To designatespeak temperature, a represents a constant and H represents a distancefrom the origin along the drawing direction where the point of thetemperature peak being elected as the origin (J. Appl. Phys., vol.49,No.8, p.4419 (1978)).

An example of Gaussian temperature distribution concerning the internaltemperature of the drawing furnace employed in experiments is shown inFIG. 4.

In the drawing furnace having the above-defined temperaturedistribution, a glass preform is subjected to drawing at the point wherethe internal temperature reaches the peak thereof, whereby the glasspreform is forced to deform largely. Then, the resultant glass which hadbeen deformed into the form of a tape is moved through the drawingfurnace while decreasing in temperature as it advances along the drawingdirection.

According to the drawing process, by making the resultant glass preformthinner, the temperature distribution of the glass along the drawingdirection can more rapidly approximate to that observed in the furnaceimmediately after the drawing, which is favorable to prevent metallichalide particles from returning to a spherical shape. For avoiding suchre-spheroidization (i.e. returning to a spherical shape), the thicknessof the glass tape which may be obtained through the drawing is suitably2 mm or less, preferably 1 mm or less.

Due to the thickness as low as 1 mm or less imparted to the drawn glass,the cooling thereof could be effected with an exceptionally highefficiency, thus succeeding in the avoidance of the re-spheroidizationof the elongated metallic halide particles. The thickness of the drawnglass may be more preferably 0.2 mm to 0.5 mm.

To avoid the re-spheroidization of metallic halide particles, it wouldpreferably be within 120 seconds, particularly within 60 seconds thatthe drawn glass preform travels from the point where the glass preformstarts to be deformed to the point that is surrounded by an atmosphereof 100° C. The above-defined duration may be modified as appropriatedepending on the thickness of the drawn glass preform. The thicker theglass is, preferably the faster it moves to be cooled in a shorter time.

The drawing ratio may be suitably determined depending on the desiredaspect ratio of metallic halide particles. The aspect ratio of metallichalide particles, in turn, may be suitably determined by considering thedesired aspect ratio of metallic particles obtained after reduction. Byway of an example, metallic particles having an aspect ratio of 2:1 to100:1 can be obtained by selecting the aspect ratio of metallic halideparticles at a value of 10:1 to 500:1.

By carrying out the drawing step under the above-defined conditions, aglass which contains metallic halide particles having a desired aspectratio and dispersed therein can be obtained. Metallic halide particleswould be subsequently reduced to metallic particles, during which thevolume of particles may decrease. Consequently, it may be preferred thatthe aspect ratio of metallic halide particles is determined byconsidering the aspect ratio of metallic particles to be obtained afterreduction.

When the end use of the resulting polarizing glass products is anoptical isolator, the above-mentioned glass preform containing metallichalide particles dispersed therein is preferably of a cross-section inthe form of rectangle or rough rectangle. The term "rough rectangle"herein includes ellipse, too. The cross-sectional shape may be suitablyselected depending on the shape required for the polarizing glass asfinal products. The term "rectangle" generally means regular square orrectangle.

The above-mentioned drawing step may be carried out preferably in such astate that a stress of 50 kg/cm² to 600 kg/cm² is applied to a glasscontaining metallic halide particles dispersed therein. The drawingunder the stress of that range makes it possible to prevent the glassfrom breaking and to elongate metallic halide particles to the desireddegree. With the stress of less than 50 kg/cm², metallic halideparticles are unable to be elongated to reach an aspect ratio of 10:1 ormore, thus is difficult to obtain a polarizing glass having itsabsorption peak wavelength in the infrared area. On the other hand, withthe stress exceeding 600 kg/cm², even when the viscosity of the glassremains in the range of 2×10⁶ to 7×10⁷ poises, the glass may be appliedwith the stress exceeding the practical maximum breaking force, thusglass breakage is likely to occur.

In the process of the present invention, it may be preferred that theglass preform subjected to drawing has at least two optically polishedfaces opposite to each other, with a view to eliminate the subsequentoptical polishing step. A couple of faces which should be opticallypolished are composed of one which behaves as light receiving face andthe other which behaves as light emitting face of a polarizing glass. Byoptically polishing faces of a glass preform prior to drawing, apolarizing glass provided with an optical plane can be obtained withoutpolishing subsequent to reduction. This is because an optical plane onthe glass surface can be maintained during the treatment includingdrawing and reduction.

An additional advantage is that a polarizing glass obtained withoutpolishing subsequent to reduction is provided with a surface suitablefor use as devices and this means that it may be possible to useprofitably the shape-anisotropic metallic particles present on the glasssurface. If cooling is carried out at the same time as drawing, theparticles present near the surface will be cooled earlier and theparticles present in the inner part of the glass will be cooled laterand thus they are more likely to return to a spherical shape. If thesurface is polished subsequent to reduction, the particles conservingthe shape-anisotropy thereof will be removed and the remaining usableparticles will be only those which had returned to a spherical shape,hence it may become more difficult to obtain a polarizing glass providedwith the desired properties. In contrast, the present invention cansolve the problem set forth above, since a polarizing glass having asurface suitable for devices can be obtained without polishingsubsequent to reduction.

The possibility of the omission of polishing subsequent to reduction canremarkably elevate the yield based on the starting material and this isalso another advantage.

The process for producing a polarizing glass article of the presentinvention may also be applicable to the production process of acomposite polarizing glass which is composed of a glass layer containingshape-anisotropic metallic particles and a base glass substratecontaining neither metallic particles nor metallic halide particles andshowing substantially no light scattering.

An example of the drawing apparatus adapted for use in the presentinvention is shown in FIG. 1.

In FIG. 1, reference numeral 1 designates a preform, and preform 1 isheld under feeding device 2 by means of a wire in a manner downwardlydisplaceable. Preform 1 is softened around the tip thereof in heatingfurnace 3 and is drawn downwardly from its bottom end by means ofpulling device 4. As the result of that drawing, the glass preform isconverted into tape-shaped glass 5 in which elongated metallic halideparticles are dispersed. Tape-shaped glass 5 continues to proceed to bedischarged out of heating furnace 3 and then quickly quenched withexternal air.

On cylindrical hood 6 situated above heating furnace 3 is attached upperlid 7 which is provided with an opening for the passage of the wirewhich connects feeding device 2 with preform 1. Hood 6 and lid 7 areeffective to prevent the glass from breaking due to a sudden increase oftemperature and to avoid the heat dispersion out of heating furnace 3.The internal temperature of heating furnace 3 is controlled by means ofa temperature control unit not shown so that the viscosity of preform 1placed in heating furnace 3 may be suitably controlled.

By controlling the feeding speed of feeding device 2 as well as thepulling rate and the tensile force applied by pulling device 4, frompreform 1, tape-shaped glass 5 could be obtained in which metallichalide particles having the desired aspect ratio are dispersed.

The resulting tape-shaped drawn glass is then subjected to reductiontreatment, during which a part or all of the metallic halide particlescontained in the glass are converted to metallic particles. Thisreduction treatment may be carried out, e.g., by thermally treating thetape-shaped glass in a reducing gas atmosphere. Suitable reducing gaseswhich may be mentioned are gaseous hydrogen, CO--CO₂ gas and the like.Reduction conditions may be modified depending on the nature of metallichalide to be reduced. The temperature for reduction should be selectedconsidering that at an excessively high temperature the resultingmetallic particles are likely to return to a spherical shape. By way ofan example, copper halide may suitably be reduced at a temperature ofabout 350° to 550° C. The duration of reduction could be properlydecided while taking together the temperature for reduction and thedesired degree of reduction into consideration and is generally selectedin the range of 30 minutes to 10 hours.

EXAMPLES

The present invention will further be described by referring to thefollowing examples.

Example 1

(1) Preparation of preform

A glass having a composition consisting of 59.9% of SiO₂, 2% of AlF₃,6.8% of Al₂ O₃, 20% of B₂ O₃, 9.7% of Na₂ O, 1% of NaCl, 0.8% of CuCland 0.1% of SnO was heated at 1410° C. in a 5-liter platinum crucibleinto a molten state, which was then cast into a mold and graduallycooled at 470° C. to form a glass block. A specimen of a desired sizewas cut out from the resulting glass block, then thermally treated at765° C. for 90 minutes to obtain a glass containing copper chlorideparticles having an average particle diameter of about 150 nm. Theresulting glass was further processed into a glass preform in the formof a plate (20 mm×2 mm×200 mm) having two optically polished facesmeasuring 20 mm×200 mm.

(2) Drawing

The preform obtained as above was subjected to drawing in the drawingapparatus shown in FIG. 1.

Preform 1 was attached to feeding device 2 (the upper end of preform 1was suspended from the lower end of feeding device 2 by means of awire). Preform 1 was positioned so that the lower end thereof may reachnear the center of heating furnace 3 (about 50 mm above the bottom endof the furnace). The internal temperature of heating furnace 3 waselevated to 710° C. by means of a temperature controlling unit notshown. (Here, the term "internal temperature" means the peak oftemperature and the peak is situated near the midpoint along the heightof the furnace. The term "internal temperature" hereinafter means thepeak of temperature.) A wire was wound around the lower end of preform 1and, after the internal temperature of furnace 3 had reached theplateau, the glass started to be elongated by putting the wire underload.

The drawn tape-shaped glass was held between the drive rollers acting aspulling device 4 and the internal temperature of heating furnace 3 wasreset at 690° C. (viscosity of the glass: ν=2×10⁷ poises; the viscositywas calculated by using viscosity curve). After the temperature hadreached the plateau, the tape-shaped glass was continuously pulled byapplying tensile force to the lower end of the glass by means of therollers. The feeding speed of feeding device 2 was 6 mm/minute, thepulling rate of pulling device 4 was 60 cm/minute and the moving period(time required for the preform to move from the point where the internaltemperature of furnace 3 reached the peak in other words, the pointwhere the preform started to be deformed to the point that wassurrounded by an atmosphere of 100° C. ) was 11 seconds.

The tensile load was 400 g, the cross section of the drawn glass wasmeasured 2 mm×0.2 mm and the stress calculated as the quotient of theload 400 g divided by the cross section was 100 kg/cm².

The term "drawing speed" herein means the distance per minute traveledby the portion having a cross section of 2 mm×0.2 mm.

The average aspect ratio of the copper chloride particles contained inthe glass was about 35:1 as observed on a transmission electronmicroscope.

(3) Reduction

The resulting tape-shaped glass was thermally treated at 425° C. for 4hours in a gaseous hydrogen atmosphere to obtain a polarizing glass(average aspect ratio of copper particles was 5:1). This polarizingglass had an extinction ratio of 50 dB (at wavelength of 1.55 μm).

Examples 2 to 6

In a manner similar to that of Example 1, five kinds of glass preformswere prepared. Each preform contained copper chloride particles havingan average particle diameter of 150 nm and had a thickness of 2.0, 5.0,8.0, 12.0 and 20.0 mm, respectively.

While viscosity of the glass was made 2×10₇ poises (viscosity of theglass at the point where the internal temperature of the furnace reachedthe peak), each of these preforms was drawn under the same conditions ofthe feeding speed, the pulling rate and the stress as those used inExample 1. Then the obtained glasses were subjected to reduction withhydrogen (at 425° C. for 4 hours) to produce polarizing glasses eachdifferent in thickness.

The results are shown in Table 1. Table 1 shows extinction ratios (atthe wavelength of 1.55 μm) and the thicknesses of the polarizingglasses.

                  TABLE 1                                                         ______________________________________                                                   Example                                                                       2      3     4         5   6                                       ______________________________________                                        Thickness of 0.2      0.5   0.8     1.2 2.0                                   polarizing glass                                                              Extinction ratio (dB)                                                                      55       53    50      48  41                                    ______________________________________                                    

The results shown in the Table 1 are further represented in FIG. 3.

It should be understood that, by making the drawn glass thinner, thecooling effect could be improved and the respheroidization could beprevented more effectively and thus a polarizing glass having a higherextinction ratio could be obtained.

In the field of optical communication, a polarizer to be incorporatedinto an isolator should be made from a polarizing glass having aspecified extinction ratio of at least 40 dB. It will be appreciatedthat a suitable polarizing glass having an extinction ratio of 40 dB ormore could be obtained by giving the drawn glass a thickness of 2 mm orless, preferably 1.0 mm or less.

Comparative Example 1

A polarizing glass was prepared in a manner similar to Example 1 exceptthat the internal temperature of the heating furnace was set at 760° C.(which imparted to the glass a viscosity of 1.3×10⁶ poises). Theobtained polarizing glass had an extinction ratio of 3 dB (at wavelengthof 1.31 μm).

This was due to the fact that the glass had a low viscosity and hencesufficiently high tensile force could not be applied upon drawing. Thestress at the drawing was 40 kg/cm².

Another polarizing glass was tried to be produced in a manner similar toExample 1 except that the internal temperature of the heating furnacewas set at 660° C. (at which the glass exhibited a viscosity of 8.3×10⁷poises). In this experiment, however, the stress became excessive at thedrawing so that the glass was broken.

Reference Example

A glass preform was prepared by holding a glass plate (20 mm×6 mm×220mm, without copper chloride particles) not subjected to the heatprecipitation treatment between a couple of glass plates (20 mm×1 mm×220mm) containing copper chloride particles having an average particlediameter of 130 nm.

The prepared preform was subjected to drawing in the apparatus shown inFIG. 1, while the temperature of the preform being kept at 690° C. (atwhich the glass exhibited a viscosity of 2×10⁷ poises, the viscosity ofthe glass at the point where the internal temperature of the furnacereached the peak). The feeding speed of feeding device 2 was 6 mm/min.,the pulling rate of pulling device 4 was 60 cm/minute, the tensile loadwas 1,600 g and the moving period (time required for the preform to movefrom the point where the internal temperature of furnace 3 reached thepeak in other words, the point where the preform started to be deformedto the point that is surrounded by an atmosphere of 100° C. ) was 11seconds.

A fused composite glass which had been heat molten at the same time asdrawn was obtained. The average aspect ratio of the copper chlorideparticles contained in the surface layer of the composite was about30:1. The resulting glass composite was subjected to reduction withhydrogen (at 425° C. for 4 hours) to obtain a polarizing glass.

This polarizing glass had an extinction ratio of 48 dB (at wavelength of1.31 g m) and an insertion loss of 0.10 dB. This was because thepolarizing glass contained no copper chloride particles in the centrallayer (substrate) thereof and thus prevented light from scattering.

According to the production process of the present invention, apolarizing glass can be prepared with a high yield from a startingmaterial of a glass containing metallic halide particles, while avoidingglass breakage or fracture during elongation as well as preventing theelongated metallic halide particles from returning to a spherical shape.

Additionally, according to the process of the present invention, apolarizing glass can be obtained from the glass preform which containsmetallic particles without relying on a coating surface glass andtherefore the time required for the reduction can be substantiallyshortened. Further, according to the production process of the presentinvention, by making the drawn glass thinner, the cooling effect can beimproved so that metallic particles can solidify while kept in theelongated state. Consequently, the metallic particles are permanentlyprevented from returning to a spherical shape and thus a polarizingglass excellent in polarization properties can be produced with a highefficiency.

What is claimed is:
 1. A process for producing a polarizing glassarticle containing shape-anisotropic metallic particles dispersed in anoriented state therein, which comprises:drawing a glass preformcontaining metallic halide particles dispersed therein to elongate saidmetallic halide particles while viscosity of the glass is held above2×10⁶, but below 7×10⁷ poises, to obtain a glass having a thickness of 2mm or less; cooling the drawn glass under conditions which avoidre-spheroidization of the elongated metallic halide particles andbreakage of the drawn glass; and subjecting the drawn glass to reducingtreatment so that a part or all of the metallic halide particles arereduced to metallic particles.
 2. The process of claim 1 wherein saiddrawing is carried out to obtain a glass having a thickness of 1 mm orless.
 3. The process of claim 1 wherein said drawing is carried out toobtain a glass having a thickness of from 0.2 to 0.5 mm.
 4. The processof claim 1 wherein said drawn glass preform travels within 120 secondsfrom the point where said glass preform starts to be deformed duringsaid drawing to the point that is surrounded by an atmosphere of 100° C.5. The process of claim 1 wherein said drawing of said glass preform iseffected under a stress of 50 kg/cm² to 600 kg/cm² inclusive.
 6. Theprocess of claim 1 wherein said drawing is effected on the glass preformhaving at least two polished faces opposite to each other.
 7. Theprocess of claim 1 wherein said drawing of said glass preform iseffected while its viscosity is held from 4×10⁶ to 4×10⁷ poises.
 8. Theprocess of claim 1 wherein said metallic halide particles are selectedfrom the group consisting of halide particles of copper, silver, goldand platinum.
 9. The process of claim 1 wherein said drawn glasspreforms is cooled at a rate ΔT until the temperature of the drawn glasspasses an annealing point, wherein ΔT≧(T1-T2)/120 sec, wherein T1 is atemperature at which the glass preform starts to be deformed during thedrawing, and T2 is a temperature of 100° C. of atmosphere surroundingthe drawn glass.